Reciprocating engines such as those for motor vehicles, motorcycles, agricultural machines, or marine vessels require a crankshaft to extract power by converting reciprocating motion of pistons to rotary motion. Crankshafts are generally categorized into two classes: those of the type produced by die forging and those of the type produced by casting. In particular, in cases where high strength and high stiffness are required, the firstly mentioned forged crankshafts, which are superior in those properties, are often employed.
In general, forged crankshafts are produced by using, as a starting material, a billet having a circular or square cross section and having a constant cross-sectional area over the entire length, and subjecting the billet to the steps of preforming, die forging, trimming and coining in order. Typically, the preforming step includes the steps of roll forming and bending, and the die forging step includes the steps of block forging and finish forging.
FIG. 1 is a schematic diagram illustrating a typical conventional process for producing a forged crankshaft. A crankshaft 1 illustrated in FIG. 1 (see FIG. 1(f)) is designed to be mounted in a 4-cylinder engine and includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange F1, and eight crank arms A1 to A8 (hereinafter also referred to simply as “crank arm”) that connect the journals J1 to J5 and the crank pins P1 to P4 to each other. The crankshaft 1 is configured such that all of the eight crank arms A1 to A8 are integrally formed with counterweights W1 to W8 (hereinafter also referred to simply as “counterweight”), respectively, and is referred to as a 4-cylinder 8-counterweight crankshaft.
Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, the crank arms A1 to A8, and the counterweights W1 to W8 are each collectively referred to, the reference character “J” is sometimes used for the journals, “P” for the crank pins, “A” for the crank arms, and “W” for the counterweights. A crank pin P and a pair of crank arms A (including the counterweights W) which connect with the crank pin P are also collectively referred to as a “throw”.
According to the production method shown in FIG. 1, the forged crankshaft 1 is produced in the following manner. Firstly, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by an induction heater or a gas atmosphere furnace and then is subjected to roll forming. In the roll forming step, the billet 2 is rolled and reduced in cross section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 3, which is an intermediate material, is formed (see FIG. 1(b)). Next, in the bend forging step, the rolled blank 3 obtained by roll forming is partially pressed from a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 4, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Subsequently, in the block forging step, the bent blank 4 obtained by bend forging is press forged with a pair of upper and lower dies, whereby a forged blank 5 including a shape generally resembling the shape of the crankshaft (end product) is formed (see FIG. 1(d)). Then, in the finish forging step, the block forged blank 5 obtained by block forging is further processed by press forging the block forged blank 5 with a pair of upper and lower dies, whereby a forged blank 6 including a shape conforming to the shape of the end product crankshaft is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out as flash from between the parting surfaces of the dies that oppose each other. Thus, the block forged blank 5 and the finish forged blank 6 have large flash 5a, 6a, respectively, around the shape of the crankshaft.
In the trimming step, the finish forged blank 6 with the flash 6a, obtained by finish forging, is held by the dies from above and below and the flash 6a is removed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the die forged crankshaft 1, from which the flash has been removed, e.g., shaft components such as the journals J, the crank pins P, the front part Fr, and the flange F1, and further the crank arms A and the counterweights W, are slightly pressed with the dies from above and below and corrected to the size and shape of the end product. In this manner, the forged crankshaft 1 is produced.
The production process shown in FIG. 1 is applicable not only for a 4-cylinder 8-counterweight crankshaft as exemplified, but also for a crankshaft in which, of all eight crank arms A, at least one of the crank arms has the counterweight W. For example, in some crankshafts to be mounted in a 4 cylinder engine, the leading first crank arm A1, the trailing eighth crank arm A8, and the two central fourth and fifth crank arms A4, A5 are each provided with the counterweight W. Such crankshafts are referred to as a 4-cylinder 4-counterweight crankshaft. The same production process can be employed for other types of crankshafts such as those to be mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type 6-cylinder engine, or an 8-cylinder engine. When adjustment of the placement angle for the crank pins is necessary, a twisting step is incorporated after the trimming step.
In recent years, there has been a need for weight reduction of reciprocating engines, particularly those for motor vehicles, in order to improve the fuel economy. Accordingly, there is also an ever-increasing demand for weight reduction of crankshafts, which are a principal component of a reciprocating engine. Conventional techniques intended for weight reduction of a forged crankshaft include the following.
Japanese Patent Application Publication No. 2012-7726 (Patent Literature 1) and Japanese Patent Application Publication No. 2010-230027 (Patent Literature 2) each disclose a crank arm having hollow portions greatly and deeply depressed toward a crank pin in the journal-side surface of the crank arm, adjacent to a straight line connecting the axis of the journal to the axis of the crank pin (hereinafter also referred to as the “crank arm centerline”), and they also each disclose a method for producing a crankshaft having the crank arm. The crank arms disclosed in Patent Literatures 1 and 2 are reduced in weight by an amount corresponding to the volumes of the hollow portions. Weight reduction of the crank win leads to a reduced weight of the counterweight, which forms a pair with the crank arm, and this in turn leads to weight reduction of the forged crankshaft as a whole. Furthermore, the crank arms disclosed in Patent Literatures 1 and 2 have sufficient stiffness (torsional rigidity and flexural rigidity) because the side portions near the crank pin, between which the crank arm centerline is interposed, have a large thickness.
By providing a recess in the journal-side surface of the crank arm while ensuring a large thickness at the side portions of the crank arm as described above, it is possible to achieve weight reduction in combination with sufficient stiffness.
However, forged crankshafts having such a unique shape are difficult to produce using conventional production methods. The reason is that, when the formation of the recess in the surface of the crank arm is to be carried out in the die forging step, a situation will occur in which the draft of the die becomes a reverse draft at the site of the recess and therefore the formed forged blank cannot be removed from the die.
To address such a situation, the production methods disclosed in Patent Literatures 1 and 2 are configured as follows: in the die forging step, the crank arm is shaped to be small without forming the recess in the surface of the crank arm, and after the trimming step, a punch is pressed into the surface of the crank arm so that the mark made by the punch forms the recess.